The Pierre Auger Collaboration has searched for the possible presence of photons with energies exceeding 1018 eV in the flux of cosmic rays arriving at Earth. These ultra-high energy photons are produced in interactions of the charged cosmic rays with energies close to 1020 eV with the cosmic microwave background (GZK effect).

When an ultrahigh-energy cosmic ray impacts the Earth’s atmosphere, it can generate an enormous cascade of billions of energetic particles capable of reaching the ground. Such a cascade is dubbed an “extensive air shower” (EAS) and is routinely being detected by the Pierre Auger Observatory in Argentina near the city of Malargüe, using an array of surface detectors deployed over 3000 km2.

One of the most pressing mysteries in astroparticle physics is the composition and origin of cosmic rays at energies around 1 EeV = 1018 eV. In contrast to charged particles, deflected in galactic and extragalactic magnetic fields, neutral particles such as neutrinos, neutrons, or photons point back to their production site. One way to learn more about sources of ultra-high energy cosmic rays is therefore to search for an accumulation of events from specific directions. This is akin to astronomical observations of a distant galaxy, say, made with an optical telescope collecting photons of visible light.